Symptoms and Signs
The diagnostic symptom of bean yellow mosaic is the bright yellow to green mosaic or mottle appearance of infected leaves, which becomes most apparent on leaves as they age. Infected leaves also show varying degrees of leaf distortion, downward cupping, and wrinkling. Plants infected at a young age may show stunted growth. The striking yellow mosaic symptoms differentiate bean yellow mosaic infections from those of bean common mosaic, which causes light and dark green mosaic patterns of infected leaves (see photo comparison).
Comments on the Disease
Bean yellow mosaic is not a common disease of beans in California. However, it has a wide host range in legumes and can readily overwinter in legume crops (e.g. alfalfa, clovers, and vetch), weeds, and gladiolus. The virus is transmitted by over 20 species of aphids (e.g., the pea, green peach, and bean aphids). Transmission of the virus to the bean plant occurs within seconds once virus-carrying aphids begin feeding on the crop. Aphids can effectively spread the virus within a field, resulting in high rates of infection. The virus is not known to be seed-transmitted in beans.
Bean yellow mosaic virus is in the same virus family as Bean common mosaic virus and both viruses commonly occur in bean fields. This overlap can confuse diagnosis, which can be particularly important in seed production fields. Antibody or DNA tests are often needed to differentiate between these two viruses.
Locate bean fields as far away from other legumes (alfalfa, clover, vetch) and gladiolus fields as possible. Insecticide sprays to reduce the rate of spread of the virus by aphids are generally not effective.